Mast-mounted vibration isolators are well-known in the art for canceling or substantially reducing vibratory forces active on a helicopter rotor. While most such devices are referred to as "vibration absorbers", this may be viewed as a misnomer inasmuch as these devices typically isolate the energy produced by cyclic in-plane and out-of-plane loads rather than absorb the energy as the name implies. Such devices typically comprise: a hub attachment fitting for mounting to the main rotor hub such that the isolator is rotated in a plane parallel to the main rotor disc, and a spring-mass system mounted to and rotating with the hub member. The spring-mass system is tuned in the non-rotating condition to a frequency equal to N * rotor RPM (e.g., 4P for a four-bladed rotor) at normal operating speed, so that in the rotating condition it will respond to both N+1 and N-1 frequency vibrations (i.e., 3P and 5P).
FIGS. 1a and 1b depict a prior art vibration isolator similar to those described and illustrated in Vincent el al. U.S. Pat. Nos. 4,145,936 and 4,225,287. As shown, the mast-mounted vibration isolator 100 includes a plurality of resilient arms (i.e., springs) 102 extending in a spaced-apart spiral pattern between a hub attachment fitting 104 and a ring-shaped inertial mass 106. More specifically, the hub attachment fitting 104 includes a stanchion 108 mounting to and rotating with a helicopter main rotor hub 110 and a mounting adapter 112 which is supported and driven by the hub-mounted stanchion 108. Several pairs of spiral springs 102 (i.e., four upper springs 102a and four lower springs 102b) are mounted to and equiangularly arranged with respect to both the hub attachment fitting 104 and the inertial mass 106 so as to produce substantially symmetric spring stiffness in an in-plane direction. Each spring 102 is comprised of unidirectional fiberglass so as to provide low in-plane bending stiffness and superior fatigue properties.
In Fig. 1b, the inertial mass 106 is comprised of three segments which include a central ring 106c and a pair of ring-shaped plates 106a, 106b mounted in combination therewith so as to produce a substantially C-shaped cross-section. As such, the C-shaped configuration provides the requisite stiffness to obviate distortion and the adverse consequences of higher harmonic dynamic resonances. Furthermore, each of the ring-shaped plates 106a, 106b define a circular abutment surface 120 which is aligned with a plurality of elastomer pads 122 which are bonded to the hub attachment fitting 104. As such, the ring-shaped plates 106a, 106b coact with one or more of the elastomer pads 122 to limit the in-plane motion of the inertial mass 106 and delimit the maximum stresses acting on the spiral springs 102.
To provide the necessary structural rigidity and properly react the motion limiting loads, the inertial mass 106 also includes a piloting arrangement 124 (see detail FIG. 1c) wherein a circular shoulder 126 is formed in each of the ring-shaped plates 106a, 106b and disposed in abutting combination a mating surface 128 of the central ring 106c. As such, radial loads are directly transferred from the shoulder 126 to the central ring 106c without imposing shear loads on the connecting fasteners 130. Finally, the upper and lower surfaces 106a.sub.s, 106b.sub.s of the ring-shaped plates 106a, 106b provide a convenient mounting location for tuning weights 134 which may be added or deleted to optimally tune the vibration isolator 100.
While the teachings disclosed in the '936 and '287 patents provide a baseline for design and development, the vibration isolator described therein, and particularly, the configuration of the inertial mass 106, is disadvantageous for various reasons. Firstly, the piloting arrangement 124 requires highly precise and costly machining of each segment of the inertial mass 106 (i.e., the shoulders 126 of the ring-shaped plates 106a, 106b and the complementary mating surfaces 128 of the central ring 106c). Secondly, it will be appreciated that the connecting fasteners 130 and the machining of the respective threaded inserts increases the fabrication costs without providing a significant structural function, i.e., other than to facilitate assembly/disassembly of the vibration isolator 100. Thirdly, the elastomer pads 122 are prone to frequent disbond and repair. Finally, by affixing the tuning weights 134 to the upper and lower surfaces 106a.sub.s, 106b.sub.s of the ring-shaped plates 106a, 106b, the height dimension of the vibration isolator 100 is increased, thereby increasing the profile area for producing aerodynamic drag.
A need, therefore, exists for an inertial mass for a vibration isolator which, inter alia, facilitates assembly/disassembly, provides improved structural efficiency, reduces fabrication costs, and ameliorates the aerodynamic drag characteristics of the vibration isolator.